Conventional single screw compressors of this kind include the one shown in FIG. 7A. This single screw compressor has a screw rotor 102 which is installed in a casing (not shown) and has spiral grooves 101, 101 . . . , a shaft 104 driving the rotation of this screw rotor 102 around its axis and two gate rotors 107, 107 which have teeth 106, 106 . . . engaged with the grooves 101, 101 . . . of the screw rotor 102 and rotate around their axes substantially perpendicular to the axis of the screw rotor 102. FIG. 7B is a cross sectional view showing the single screw compressor in a plane including the axis of the screw rotor 102, and shows the screw rotor 102 and one gate rotor 107 of the two gate rotors 107 engaged with the screw rotor 102. When rotation of the screw rotor 102 is driven by the shaft 104 as shown with arrow A in FIG. 7A, the gate rotors 107, 107 rotate in a direction shown with arrow B. Consequently, the volume of compression spaces partitioned by an inner surface of the casing (not shown), the grooves 101 of the screw rotor and the teeth 106 of the gate rotors are reduced and hence gases introduced into the compression spaces are compressed.
The number of the grooves 101 of the screw rotor 102 is six, and the number of the teeth 106 of the gate rotor 107 is eleven. Since six, which is the number of the grooves 101, and eleven, which is the number of the teeth 106, are relatively prime, all the teeth 106, 106 . . . are each engaged with all the grooves 101, 101 . . . when this single screw compressor is operated.
However, since all the teeth 106, 106 . . . of the gate rotor 107 are each engaged with all the grooves 101, 101 . . . of the screw rotor 102, the conventional single screw compressor needs to be formed so that any of the teeth 106 of the gate rotor 107 can be engaged with a groove 101 having the smallest dimension in the screw rotor 102. That is, the largest tooth 106 dimension in the gate rotor 107 needs to be made smaller than the smallest groove 101 dimension in the screw rotor 102. Consequently, when a tooth 106 of the gate rotor 107 having the smallest dimension is engaged with a groove 101 of the screw rotor 102 having the largest dimension, a clearance between the groove 101 and the tooth 106 becomes large, and a problem arises that a gas to be compressed leaks. In order to prevent this gas leakage, the gate rotors 107 and the screw rotor 102 need to be processed in high accuracy with an extremely small dimensional tolerance so that the clearance between the teeth 106 and the grooves 101 becomes small. As a result, costs for processing the gate rotors 107 and the screw rotor 102 become high, and hence costs for manufacturing the single screw compressor become high.